The thing is, there really is no time-traveling element if they stick to the storyline from the books. It should be two different timelines with one character being transplanted in to the body of her/his younger self. If they use Wanda (Scarlet Witch) or Jean to do that then great but there should be no time travel

that kinda limits who can be used then... if there going from the present to the 60's, the only characters that would be old enough to have a younger counterpart back then, are Xavier, Beast, Magneto, Mystique and Wolverine